This invention relates generally to a valve and, more particularly, to a pilot-operated, spring-centered, control valve. The invention is also directed to the employment of such a control valve in a circuit-controlling operation of a hydrostatic front wheel drive system for a vehicle.
The invention is concerned with a sliding spool-type directional control valve in which a valve spool slides axially in a bore formed in a valve body to control the direction of hydraulic fluid flow therethrough. Such spool valves are often remotely controlled by the use of pilot pressure which is applied at the ends of the spool. Spool valves may also be spring-centered so that springs located at either end of the spool bias the spool to a centered position within the bore. The spool is held in its normal centered position by the springs and shifted to other offset positions by the application or removal of pilot pressure at the ends of the spool. Walters U.S. Pat. No. 3,763,746 and Finley U.S. Pat. No. 3,557,829 disclose directional control valves which are spring-centered and pilot-controlled.
During operation of a spool valve, the spool may be shifted from one offset position on one side of its centered position immediately to an offset position on the other side. The springs will cause the spool to move to a centered position, but a pressure differential is required to move the spool beyond the centered position to a new offset position. In spool valves of conventional design, pilot pressure does not move the valve spool rapidly enough to prevent a noticeable time lag between the application or removal of pilot pressure and movement of the spool.
Some pilot-operated directional control valves employ a differential pilot piston to actuate the valve spool. Since the speed of a pilot-operated spool or piston is a function of its effective cross-sectional area and the pressure applied to that effective area, the use of a piston to drive a spool of differing size will proportionally modify spool speed for any given amount of applied pilot pressure.
While it is intended that the control valve be employed in an environment such as that shown in Kress et al. U.S. Pat. No. 3,481,419, Nighswonger et al. U.S. Pat. No. 3,480,099 and Malm et al. U.S. Pat. No. 3,458,005, which are illustrative of hydrostatic front wheel drive systems for vehicles having hydraulic drive motors, pressure-actuated clutches, and means for series or parallel mode drive, the control valve shown herein may be employed in other hydraulic circuits in which immediate response to pilot pressure is required.